In some operations it is necessary to reduce minerals to a comparitively fine particle size. This can be achieved in a number of ways but with a centrifugal accelerating rotor it cannot be achieved satisfactorily in a single pass through the apparatus. While the material can be reconveyed to the inlet for reprocessing this is costly and not particularly effective with the normal centrifugal type of mineral breaker. A mineral breaker of the type referred to has been described and claimed in U.S. Pat. No. 3,970,257.